U.S. and European diplomats said Thursday that they were pleased with a new, more positive approach by Iran in talks aimed at resolving the impasse over its nuclear program. They set a new round of negotiations for next month in Geneva.

After a group meeting and then a one-on-one session between Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif, Kerry called the talks "constructive" and said he was struck by a "very different tone" from Iran. However, he emphasized that words must be translated into action if Iran wants to prove it is not seeking to develop a nuclear weapon.

"We've agreed to try to continue a process that would try to make concrete and find a way to answer the questions that people have about Iran's nuclear program," Kerry said. "Needless to say, one meeting and a change in tone, that was welcome, does not answer those questions."

"All of us were pleased that the foreign minister came today and that he did put some possibilities on the table,'' Kerry said.

The meeting between Zarif and Kerry, who sat next to each other at a U-shaped table, was the highest-level direct contact between the United States and Iran in years and, in terms of substantive discussions, the first in a generation.

Iran and the United States have been at odds since Iran's 1979 Islamic Revolution toppled the U.S.-backed shah. The last time a U.S. secretary of state met with Iran's foreign minister was in May 2007, when Condoleezza Rice made clear she was open to talking to her Iranian counterpart, Manouchehr Mottaki, at an international conference in Egypt, but the encounter amounted largely to pleasantries over ice cream, Reuters reported.

European Union foreign policy chief Catherine Ashton said Kerry and Zarif shook hands and were cordial. She called the meeting "substantial" and announced that the parties had agreed to "go forward with an ambitious timeframe." The next step will be a meeting of senior negotiators in Geneva on Oct 15 and 16, she said.

Speaking after Kerry, Zarif said the meetings had been "very constructive" and "very businesslike."

"We hope to be able to make progress to solve this issue in a timely fashion (and) to make sure (there is) no concern that Iran's program is anything but peaceful," he said.

"I am satisfied with this first step," Zarif said. "Now we have to see whether we can match our positive words with serious deeds so we can move forward."

He said the result would have to include "a total lifting" of the international sanctions that have devastated Iran's economy.

British Foreign Secretary William Hague said there had been a "big improvement in the tone and spirit" from the top Iranian diplomat compared with representatives of the previous Iranian government.

German Foreign Minister Guido Westerwelle said the meeting had taken place in a "completely different tone, atmosphere and spirit" than the group was used to and that a "window of opportunity has opened" for a peaceful resolution of the situation. He warned, though, that Iran's words would have to be matched by actions.

"Words are not enough," he said. "Actions and tangible results are what counts. The devil is in the details, so it is now important that we have substantial and serious negotiations very soon."

The meeting on the sidelines of the United Nations General Assembly was the highest-level direct contact between the United States and Iran in six years.

In Washington before the meeting, the White House resisted putting a timeline on the nuclear negotiations.

"We're not expecting any breakthrough in this initial meeting," White House spokesman Jay Carney said. "This is part of us testing the seriousness of the Iranians, who are obviously engaging in new overtures and showing new interest in trying to solve this very serious matter."

President Hasan Rouhani's pronouncements at the U.N. have raised guarded hopes that progress might be possible, but they have also served as a reminder that the path to that progress will not be quick or easy.

In his speech to world leaders at the U.N. on Tuesday, he repeated Iran's long-standing demand that any nuclear agreement must recognize the country's right under international treaties to continue enriching uranium.

Rouhani said earlier Thursday that all nations, including Israel, should dismantle their nuclear weapons — words that were taken as introducing obstacles to a nuclear deal.

Those comments show that Rouhani is not serious, said Michael Rubin, a former Middle East expert at the Pentagon under President George W. Bush.

"The more you complicate the issue, the more you're setting up the talks to fail," Rubin said.

Michael Doran, a former Middle East adviser in the Bush White House, said Rouhani's words about Israel are "a wise negotiating strategy" to present Iran as a victim of a Western double standard and attempt to change the subject from its own weapons.

U.S. negotiators should define "a speedy and meaningful process by which the Iranians would have to talk about brass tacks very quickly," Doran said. "The Iranian strategy will be to put all kinds of other things on the table other than reprocessing" of uranium fuel.

Rouhani's chief goal is a lifting of sanctions that have squeezed the Iranian economy, which many analysts say have brought Rouhani to the negotiating table.

"I would hope the Americans will not fall for that," Doran said.

After addressing the Council on Foreign Relations, Rouhani took a question about how many centrifuges would be needed for what he says is a peaceful nuclear program, The question has also been raised by analysts who fear the industrial-level nuclear fuel production that Iran has built will allow it to produce enough highly enriched uranium for weapons.

Rouhani directed his questioner to the website of the United Nations nuclear watchdog, the International Atomic Energy Agency. "I don't think the actual number will mean much to our listeners," he said.